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fix(ds4): correct reasoning effort prefix mapping (high unreachable, max rendered high) - #231

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fix(ds4): correct reasoning effort prefix mapping (high unreachable, max rendered high)#231
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ktsaou:fix/ds4-reasoning-effort-prefix

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@ktsaou

@ktsaou ktsaou commented Aug 3, 2026

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Problem

REASONING_EFFORT_MAX in vllm/tokenizers/deepseek_v4_encoding.py holds the HIGH prefix text from the checkpoint's reference encoder (encoding/encoding_dsv4.py), and a prefix is injected only when reasoning_effort == 'max'. Result on current main:

  • reasoning_effort=highno prefix at all (renders identically to unset/low)
  • reasoning_effort=max → renders the HIGH prefix ("Absolute maximum with no shortcuts permitted…") instead of the true max prefix ("Beyond maximum — exhaustive, relentless, and uncompromising…")

So every request runs one effort level below what was requested. Agentic workloads asking for max have actually been getting high since the 0731 checkpoint introduced the third effort level.

Measured via /tokenize on a live server (r16 image line): low=5, high=5, max=84 tokens. Expected per the reference encoder: low=empty, high=84, max≈97.

Independently reported against stock vLLM: https://huggingface.co/deepseek-ai/DeepSeek-V4-Flash-0731/discussions/26 (byte-identical comparison against encoding/encoding_dsv4.py; notes the same high/max finding in stock tokenizer_mode=deepseek_v4, so upstream vLLM likely wants the equivalent fix).

Fix

  • Rename the existing constant to REASONING_EFFORT_HIGH (its text is the reference encoder's high prompt verbatim).
  • Add REASONING_EFFORT_MAX with the reference encoder's true max prompt, verbatim from encoding/encoding_dsv4.py in deepseek-ai/DeepSeek-V4-Flash-0731.
  • Inject the prefix for both high and max.

Validation

Patched file bind-mounted over a running production server:

  • reasoning_effort=high → 84 tokens, ends "…no assumption is left unchecked." ✓
  • reasoning_effort=max → 97 tokens, ends "…no error remains undiscovered." ✓
  • unset effort → 84 (server default high) — unchanged ✓
  • none → chat mode — unchanged ✓

Note: tokenizers/deepseek_v4.py maps lowhigh via its else-branch. That mapping looks intentional (the fork exposes none/high/max) and is left untouched, but flagging it: the reference encoder has a distinct low tier (thinking mode, empty prefix) that the fork currently doesn't expose.

Summary by CodeRabbit

  • New Features

    • Added support for a dedicated high reasoning effort mode.
    • Updated thinking-mode rendering to apply distinct prompts for high and maximum reasoning effort settings.
  • Bug Fixes

    • Improved maximum reasoning effort prompt wording for more consistent behavior.

…max rendered high)

REASONING_EFFORT_MAX held the HIGH prefix text from the checkpoint's
reference encoder (encoding/encoding_dsv4.py) and a prefix was injected
only for effort == 'max'. As a result high rendered with no prefix at
all (identical to none/low) and max silently rendered the high prompt
("Absolute maximum with no shortcuts permitted...") instead of the
true max prompt ("Beyond maximum - exhaustive, relentless, and
uncompromising..."), so every request ran one effort level below the
requested one.

Rename the existing constant to REASONING_EFFORT_HIGH, add the true
REASONING_EFFORT_MAX text verbatim from encoding/encoding_dsv4.py in
deepseek-ai/DeepSeek-V4-Flash-0731, and inject for both high and max.

Verified live via /tokenize on a running server: high=84 tokens ending
"...no assumption is left unchecked", max=97 tokens ending "...no
error remains undiscovered". Behavior for none/chat and unset effort
is unchanged. The low -> high mapping in tokenizers/deepseek_v4.py is
intentional and untouched.
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📒 Files selected for processing (1)
  • vllm/tokenizers/deepseek_v4_encoding.py
📝 Walkthrough

Walkthrough

The DeepSeek V4 tokenizer adds a high-effort reasoning prompt, revises the maximum-effort prompt, and selects the correct prompt for "high" and "max" modes.

Changes

DeepSeek reasoning effort

Layer / File(s) Summary
Reasoning prompts and selection
vllm/tokenizers/deepseek_v4_encoding.py
Adds REASONING_EFFORT_HIGH, revises REASONING_EFFORT_MAX, and maps "high" and "max" to their respective prompts.

Estimated code review effort: 1 (Trivial) | ~5 minutes

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Check name Status Explanation
Description Check ✅ Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check ✅ Passed The title clearly identifies the DeepSeek V4 reasoning-effort prefix mapping fix, including the unreachable high mapping and incorrect max rendering.
Docstring Coverage ✅ Passed No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check ✅ Passed Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check ✅ Passed Check skipped because no linked issues were found for this pull request.
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Actionable comments posted: 1

🧹 Nitpick comments (1)
vllm/tokenizers/deepseek_v4_encoding.py (1)

274-275: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Update the effort-selection comment.

The branch now handles both "high" and "max", but the preceding comment still mentions only max effort. Update it to describe both modes.

🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@vllm/tokenizers/deepseek_v4_encoding.py` around lines 274 - 275, Update the
comment immediately preceding the effort-selection branch to describe both
"high" and "max" reasoning-effort modes, matching the conditions handled by the
existing thinking_mode and reasoning_effort logic.
🤖 Prompt for all review comments with AI agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@vllm/tokenizers/deepseek_v4_encoding.py`:
- Around line 70-81: Wrap the long string literals in REASONING_EFFORT_HIGH and
REASONING_EFFORT_MAX using adjacent string segments so every line stays within
88 characters. Preserve the rendered prompt text exactly, including spaces,
punctuation, newline escapes, and paragraph spacing.

---

Nitpick comments:
In `@vllm/tokenizers/deepseek_v4_encoding.py`:
- Around line 274-275: Update the comment immediately preceding the
effort-selection branch to describe both "high" and "max" reasoning-effort
modes, matching the conditions handled by the existing thinking_mode and
reasoning_effort logic.
🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

  • Push a commit to this branch (recommended)
  • Create a new PR with the fixes

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Reviewing files that changed from the base of the PR and between e12b91b and 4ec6f72.

📒 Files selected for processing (1)
  • vllm/tokenizers/deepseek_v4_encoding.py

Comment on lines +70 to +81
REASONING_EFFORT_HIGH = (
"Reasoning Effort: Absolute maximum with no shortcuts permitted.\n"
"You MUST be very thorough in your thinking and comprehensively decompose the problem to resolve the root cause, rigorously stress-testing your logic against all potential paths, edge cases, and adversarial scenarios.\n"
"Explicitly write out your entire deliberation process, documenting every intermediate step, considered alternative, and rejected hypothesis to ensure absolutely no assumption is left unchecked.\n\n"
)

REASONING_EFFORT_MAX = (
"Reasoning Effort: Beyond maximum \u2014 exhaustive, relentless, and uncompromising.\n"
"You MUST reason with the utmost depth and rigor, leaving absolutely nothing to chance: exhaustively decompose the problem into its most fundamental components, trace every causal chain to its root, and resolve the underlying cause rather than any surface symptom.\n"
"Do not stop reasoning until you have independently verified the solution from multiple angles and are certain that no assumption remains unchecked and no error remains undiscovered.\n\n"
)

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📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

Wrap the long prompt literals.

Lines 72, 73, 78, and 79 exceed the 88-character limit. Split the literals with adjacent strings. Preserve all spaces and escape sequences so the rendered prompts do not change.

Proposed fix
-    "You MUST be very thorough in your thinking and comprehensively decompose the problem to resolve the root cause, rigorously stress-testing your logic against all potential paths, edge cases, and adversarial scenarios.\n"
-    "Explicitly write out your entire deliberation process, documenting every intermediate step, considered alternative, and rejected hypothesis to ensure absolutely no assumption is left unchecked.\n\n"
+    "You MUST be very thorough in your thinking and comprehensively decompose the "
+    "problem to resolve the root cause, rigorously stress-testing your logic "
+    "against all potential paths, edge cases, and adversarial scenarios.\n"
+    "Explicitly write out your entire deliberation process, documenting every "
+    "intermediate step, considered alternative, and rejected hypothesis to ensure "
+    "absolutely no assumption is left unchecked.\n\n"
...
-    "You MUST reason with the utmost depth and rigor, leaving absolutely nothing to chance: exhaustively decompose the problem into its most fundamental components, trace every causal chain to its root, and resolve the underlying cause rather than any surface symptom.\n"
-    "Do not stop reasoning until you have independently verified the solution from multiple angles and are certain that no assumption remains unchecked and no error remains undiscovered.\n\n"
+    "You MUST reason with the utmost depth and rigor, leaving absolutely nothing to "
+    "chance: exhaustively decompose the problem into its most fundamental components, "
+    "trace every causal chain to its root, and resolve the underlying cause rather "
+    "than any surface symptom.\n"
+    "Do not stop reasoning until you have independently verified the solution from "
+    "multiple angles and are certain that no assumption remains unchecked and no "
+    "error remains undiscovered.\n\n"

As per coding guidelines, Python code must follow an 88-character line length limit.

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
REASONING_EFFORT_HIGH = (
"Reasoning Effort: Absolute maximum with no shortcuts permitted.\n"
"You MUST be very thorough in your thinking and comprehensively decompose the problem to resolve the root cause, rigorously stress-testing your logic against all potential paths, edge cases, and adversarial scenarios.\n"
"Explicitly write out your entire deliberation process, documenting every intermediate step, considered alternative, and rejected hypothesis to ensure absolutely no assumption is left unchecked.\n\n"
)
REASONING_EFFORT_MAX = (
"Reasoning Effort: Beyond maximum \u2014 exhaustive, relentless, and uncompromising.\n"
"You MUST reason with the utmost depth and rigor, leaving absolutely nothing to chance: exhaustively decompose the problem into its most fundamental components, trace every causal chain to its root, and resolve the underlying cause rather than any surface symptom.\n"
"Do not stop reasoning until you have independently verified the solution from multiple angles and are certain that no assumption remains unchecked and no error remains undiscovered.\n\n"
)
REASONING_EFFORT_HIGH = (
"Reasoning Effort: Absolute maximum with no shortcuts permitted.\n"
"You MUST be very thorough in your thinking and comprehensively decompose the "
"problem to resolve the root cause, rigorously stress-testing your logic "
"against all potential paths, edge cases, and adversarial scenarios.\n"
"Explicitly write out your entire deliberation process, documenting every "
"intermediate step, considered alternative, and rejected hypothesis to ensure "
"absolutely no assumption is left unchecked.\n\n"
)
REASONING_EFFORT_MAX = (
"Reasoning Effort: Beyond maximum \u2014 exhaustive, relentless, and uncompromising.\n"
"You MUST reason with the utmost depth and rigor, leaving absolutely nothing to "
"chance: exhaustively decompose the problem into its most fundamental components, "
"trace every causal chain to its root, and resolve the underlying cause rather "
"than any surface symptom.\n"
"Do not stop reasoning until you have independently verified the solution from "
"multiple angles and are certain that no assumption remains unchecked and no "
"error remains undiscovered.\n\n"
)
🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@vllm/tokenizers/deepseek_v4_encoding.py` around lines 70 - 81, Wrap the long
string literals in REASONING_EFFORT_HIGH and REASONING_EFFORT_MAX using adjacent
string segments so every line stays within 88 characters. Preserve the rendered
prompt text exactly, including spaces, punctuation, newline escapes, and
paragraph spacing.

Source: Coding guidelines

@ktsaou

ktsaou commented Aug 3, 2026

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@coderabbitai addressed in aa2f68e — the comment now covers both high and max effort injection.

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@ktsaou Confirmed. The update covers both high and max effort prefix injection.


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The reasoning-prefix bug reported here is valid. A GG-complete successor is now available in #235.

#235 retains the corrected official high/max prompts and additionally covers the official low tier, Python/Rust renderer parity, message-level system tools, request-level system/tool ordering, and golden tests against the 0731 encoder. I am leaving this PR open for its author/maintainers rather than closing it automatically.

@ktsaou

ktsaou commented Aug 4, 2026

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Thanks for confirming and for the quick turnaround. Closing this in favor of #235, which covers everything here plus the low tier, Rust parity, ordering, and golden tests. For anyone landing here later: the same bug class exists in stock vLLM's tokenizer_mode=deepseek_v4 path; the upstream track is vllm-project#50580.

@ktsaou ktsaou closed this Aug 4, 2026
JMPSequeira pushed a commit to JMPSequeira/vllm that referenced this pull request Aug 19, 2026
…registry

Port of the production CKV prefetch layer-cache poisoning fix (702d7fe in
the overlay tree) onto this branch, adapted to this branch's formats.

The registration site inside the CKV gather path registered ``kv_cache``,
which for KVarN MLA prefill is the FP8 staging view returned by
_stage_kvarn_mla_fp8_cache. That workspace is deliberately shared across
ALL MLA layers (one dense arena keyed by device/pages/geometry), while the
record it holds is materialized from the registering layer's own paged
slice. Registering it therefore poisons every ``layer_caches`` entry: the
next gather-eligible request's side-stream prefetches then gather all
prefetched layers from that one tensor, feeding every prefetched layer a
previous layer's KV (in production this was measured as an identical
wrong byte checksum for every layer's chunk-0 gather from a single kv
pointer, corrupting attention for every layer >= 1).

The production fix registers the real per-layer paged cache; that tensor
is directly gatherable there because production carries a native KVarN
CKV gather over the packed paged record. This branch's _dcp_gather_ckv
only accepts the 656-byte staged layout, so the faithful minimal fix is
to keep KVarN MLA out of the registry entirely: prefetched layers get no
registry entry, the target chain stops, and every gather stays on the
synchronous per-layer path (the same path the first eligible request
already takes). Non-KVarN formats still register their own per-layer
paged view, which is unchanged and correct.

The full registration fix (register the per-layer paged cache plus the
native packed-record gather) lands with the b12x production backend (PR
local-inference-lab#231).

Signed-off-by: João Sequeira <email.sequeira@gmail.com>
JMPSequeira pushed a commit to JMPSequeira/vllm that referenced this pull request Aug 19, 2026
…registry

Port of the production CKV prefetch layer-cache poisoning fix (702d7fe in
the overlay tree) onto this branch, adapted to this branch's formats.

The registration site inside the CKV gather path registered ``kv_cache``,
which for KVarN MLA prefill is the FP8 staging view returned by
_stage_kvarn_mla_fp8_cache. That workspace is deliberately shared across
ALL MLA layers (one dense arena keyed by device/pages/geometry), while the
record it holds is materialized from the registering layer's own paged
slice. Registering it therefore poisons every ``layer_caches`` entry: the
next gather-eligible request's side-stream prefetches then gather all
prefetched layers from that one tensor, feeding every prefetched layer a
previous layer's KV (in production this was measured as an identical
wrong byte checksum for every layer's chunk-0 gather from a single kv
pointer, corrupting attention for every layer >= 1).

The production fix registers the real per-layer paged cache; that tensor
is directly gatherable there because production carries a native KVarN
CKV gather over the packed paged record. This branch's _dcp_gather_ckv
only accepts the 656-byte staged layout, so the faithful minimal fix is
to keep KVarN MLA out of the registry entirely: prefetched layers get no
registry entry, the target chain stops, and every gather stays on the
synchronous per-layer path (the same path the first eligible request
already takes). Non-KVarN formats still register their own per-layer
paged view, which is unchanged and correct.

The full registration fix (register the per-layer paged cache plus the
native packed-record gather) lands with the b12x production backend (PR
local-inference-lab#231).

Signed-off-by: João Sequeira <email.sequeira@gmail.com>
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